1. Field of the Invention
This invention relates to an image pickup apparatus, such as a video camera or the like, having an automatic focusing device and arranged to permit selection between a moving-image shooting mode and a still-image shooting mode.
2. Description of the Related Art
The automatic focusing device included in a video camera or the like is arranged to control a focusing action on the basis of a video signal coming from an image sensor for accurate focusing, to permit setting a focus detecting area as desired and to prevent focus variations and occurrence of a parallax from being caused by zooming, The automatic focusing device can be simply arranged with no special sensor additionally provided for focusing.
FIG. 1 shows in a block diagram the arrangement of the conventional automatic focusing device of the above-stated kind. Referring to FIG. 1, a light signal obtained by a focusing lens 1 from an object of shooting is imaged on an image sensor 2 disposed to the rear of the focusing lens 1. A video signal is outputted from the image sensor 2. A luminance signal obtained from the video signal is inputted to a band-pass filter 3 which is connected to the image sensor 2. The band-pass filter 3 extracts from the luminance signal a predetermined high frequency component which varies according to the state of focus. The high frequency component is inputted to a detection circuit 4 which is connected to the band-pass filter 3. The detection circuit 4 detects the amplitude of the high frequency component inputted (integrated). The result of the detection is taken out as a focus detection signal. The focus detection signal is inputted to a gate circuit 5 which is connected to the detection circuit 4. The gate circuit 5 extracts from the focus detection signal only a portion of the focus detection signal obtained from a focus detecting area set within an image plane. The focus detection signal thus extracted through the gate circuit 5 is inputted to an evaluation value computing circuit 6 which is connected to the gate circuit 5. The evaluation value computing circuit 6 computes either the integrated value or the differentiated value of the focus detection signal for each field to obtain a focus evaluating signal for the current field. To the evaluation value computing circuit 6 is connected an in-focus decision circuit 8 which is arranged to decide an in-focus state. A lens control circuit 9 is connected to the in-focus decision circuit 8 and is arranged to control the position of the focusing lens 1. A motor 12 which is arranged to drive the focusing lens 1 is connected to the lens control circuit 9 through a lens driving circuit 10. To the motor 12 is connected a lens position detecting circuit 11 which detects the position of the focusing lens 1. The lens position detecting circuit 11 is connected to the lens control circuit 9. A memory 7 is connected to the in-focus decision circuit 8 and the lens control circuit 9.
The automatic focusing device which is arranged in the above-stated manner performs a focusing action in accordance with a so-called hill-climbing method. At the start of the focusing action, an optical path length is varied, for example, by slightly vibrating the focusing lens 1 or the image sensor 2 in the direction of an optical axis. The in-focus decision circuit 8 computes the direction in which a defocused state takes place by detecting changes in a focus evaluating signal. Then, the lens driving circuit 10 drives the motor 12 to cause the focusing lens 1 to be moved in the direction in which the focus evaluating signal becomes larger. In this case, the focusing lens 1 is moved in one and the same direction as long as the focus evaluating signal of the current field is judged to be larger than the focus evaluating signal of a preceding field. The largest value of the focus evaluating signals obtained up to now and the position of the focusing lens 1 obtained when the focus evaluating signal is at the largest value are stored in the memory 7. Further, even when the focus evaluating signal of the current field is judged to be smaller than the focus evaluating signal of the preceding field, if the focus evaluating signal of the current field is larger than a predetermined percent of the maximum value of the focus evaluating signal obtained up to that time, the focusing lens is driven in the same direction. If the focus evaluating signal becomes smaller than the predetermined percent, the focusing lens is regarded as having passed over an in-focus point and the driving direction of the focusing lens 1 is reversed. Then, the focusing lens is moved to the lens position corresponding the maximum value of the focus evaluating signal stored in the memory 7, so that the focusing action is completed. The in-focus decision circuit 8 generates a coincidence signal when the focus evaluating signal obtained from the focusing lens 1 which is reversely moved comes to coincide with the largest value of the focus evaluating signal stored in the memory 7. In response to the coincidence signal, the lens control circuit 9 issues a stop instruction to bring the focusing lens 1 to a stop.
FIG. 2 is a graph showing the characteristic of a relation obtained between the position of the focusing lens and the focus evaluating signal. The peak value Vf of the focus evaluating signal which is obtained when the lens is judged to be in an in-focus state is stored in the memory 7. Even after the focusing action is completed and the focusing lens is stopped, the focus evaluating signal continues to be detected. If the focus evaluating signal becomes smaller than a restart threshold value Vth, the movement of the focusing lens is restarted. Accordingly, the restart threshold value Vth is determined in a predetermined ratio to the peak value Vf. The focusing accuracy can be increased by having the restart threshold value Vth set at a high level. If the restart threshold value Vth is set at a high level, however, the focusing, action would be restarted even when the luminance of the object of shooting slightly changes or even when another object happens to move past the object. In such a case, the image plane fluctuates to give a disagreeable impression in viewing the video image. Thus, in the video camera, the continuity of video images is regarded as more important than the momentary image quality (focusing and exposure). To prevent the focusing action from being unnecessarily restarted in such a manner, therefore, the restart threshold value Vth is set in consideration of the diameter of an allowable circle of confusion of the image pickup system of the video camera. Unless the focus evaluating signal obtained during shooting becomes smaller than the restart threshold value Vth, the motor 12 is stopped to keep the focusing lens 1 to its position (an in-focus point) obtained when the peak value Vf which is stored in the memory 7 is obtained.
Further, in the video camera, the iris is set in such a manner to make the responsivity relative to a change of brightness slow. Therefore, a change of the picked-up image is made moderate with respect to a sudden change of brightness, so that the brightness of the image is stabilized.
The automatic focusing device used for the conventional video camera is arranged as described above. Meanwhile, there has been proposed a video camera which is arranged to be capable of operating not only in a moving-image shooting mode but also in a still-image shooting mode in which a shot of a still image can be taken and to permit switching between these two modes.
In the moving-image shooting mode, the focus is continuously adjusted until an in-focus state is attained. However, in the moving-image shooting mode, since the continuity of video images is regarded as more important than the accuracy of focusing, the video camera of the above-stated kind is arranged to continue a shooting operation and keep the focusing lens stopped even in an out-of-focus state as long as the focus evaluating signal is not found to be smaller than the restart threshold value Vth. In other words, the restart threshold value Vth is set as low as possible to give priority to the continuity of video images. On the other hand, in an ordinary still-image-dedicated shooting camera, importance is attached to a momentary high quality of an image taken in. Therefore, no exposure is allowed with the lens out of focus. A shutter release is made only after an in-focus state is attained.
In shooting moving images, therefore, importance is attached to the continuity of video images and, if the amount of blur of the images is about the same as the diameter of an allowable circle of confusion, the focusing action is not restarted. The stopped position of the focusing lens in shooting moving images is not always a position at which the peak value of the focus evaluating signal is obtained.
In the event of shooting a still image, since importance is attached to a momentary image quality, it is necessary to have a main object of shooting approximately in the middle of the depth of field and to have about the same amount of blur in front and in rear of the main object. Further, in order to avoid any out-of-focus shooting for obtaining a high quality image, any defocused state caused by an error of the stopping position of the focusing lens must be minimized. It is, therefore, preferable to make a shutter release with the focusing lens in a position corresponding to the peak value of the focus evaluating signal.
Therefore, it has been necessary for the video camera of the kind arranged to permit switching between a moving-image shooting mode and a still-image shooting mode either to have two focusing devices arranged separately for the moving-image shooting mode and the still-image shooting mode or to have one focusing device arranged in common for both of the shooting modes at the expense of quality of its focusing characteristic of one of these shooting modes. In other words, in the latter case, the video camera of the above-stated kind has been arranged not to restart the focusing action unless the value of the focus evaluating signal becomes less than the restart threshold value Vth even in the still-image shooting mode.
With the focusing action arranged to be carried out in the hill climbing method mentioned above, the focusing accuracy depends on the aperture value of the image pickup system, i.e., on the depth of field. When the aperture value (F-number) is large (when the size of the aperture of the iris is small), the hill-like shape of the focus evaluating signal (shown in FIG. 2) becomes gently sloping near the in-focus position. The depth of focus becomes deeper to make detection of the peak point of the focus evaluating signal difficult and to lower the focusing accuracy. Further, when the aperture value (F-number) becomes larger, i.e., when the size of the aperture of the iris becomes smaller, the quantity of incident light decreases to lower the value of the focus evaluating signal. The focusing accuracy decreases under such a condition as it becomes difficult to detect an in-focus point like in the case of shooting a low luminance object. More specifically, as the aperture value (F-number) is smaller, i.e., when the size of the aperture of the iris is larger, the depth of field becomes shallower, so that focus detection can be more accurately performed. Accordingly, the hill-climbing AF action has a maximum accuracy when the iris is fully opened. In the event of a low focusing accuracy, the conventional automatic focusing device has been arranged to carry out automatic focusing with aperture values set according to shooting conditions.
In the video camera arranged as described above to be capable of having the moving-image shooting mode and the still-image shooting mode, the restart threshold value Vth is set to be used in common for both the moving-image shooting mode and the still-image shooting mode. It is, therefore, sometimes hardly possible to have an optimum focusing accuracy in each of these shooting modes. For example, in carrying on shooting by switching the moving-image shooting mode over to the still-image shooting mode, when the amount of blur obtained immediately before the end of shooting in the moving-image shooting mode is within the range of allowable values determined by the restart threshold value Vth, the shooting comes to be performed in the still-image shooting mode under the same condition without performing the focusing action again. The quality of a still image thus obtained becomes the same as the image quality obtained in the moving-image shooting mode. In such a case, the focusing accuracy is too low for a still image and gives a low image quality.
In the video camera mentioned above, an in-focus state is accurately attainable in the still-image shooting mode, if the focusing action is performed by setting the aperture value (F-number) at a small value (making the size of the aperture of the iris large) for accurately setting a focusing condition to make the peak position of the focus evaluating signal easily detectable. However, it is difficult to employ such a method, because, in the moving-image shooting mode in which video images are to be continuously picked up, the quantity of light apposite to the focusing action often does not coincide with the quantity of light apposite to shooting and recording.
This invention is developed in view of the problems in the video camera of the kind mentioned in the foregoing. It is, therefore, an object of this invention to provide a video camera which permits selection of a shooting mode from between moving-image shooting and still-image shooting, has an automatic focusing device and is capable of enhancing the quality of still images obtained in the still-image shooting mode.
To attain this object, an image pickup apparatus arranged as a preferred embodiment of this invention includes switching means for switching between a moving-image shooting mode and a still-image shooting mode, automatic focusing means for carrying out a focusing action by detecting a high frequency component of a video signal as a focus evaluating signal and by driving an image pickup system on the basis of the focus evaluating signal, and restart means for restarting the focusing action by controlling the automatic focusing means when the moving-image shooting mode is switched to the still-image shooting mode by the switching means.
An image pickup apparatus which is arranged as another preferred embodiment of this invention includes switching means for switching between a moving image shooting mode and a still-image shooting mode, automatic focusing means for carrying out a focusing action by detecting a high frequency component of a video signal as a focus evaluating signal and by driving an image pickup system on the basis of the focus evaluating signal, and control means for changing over a characteristic of the focusing action of the automatic focusing means correspondingly with the moving-image shooting mode or the still-image shooting mode selected through the switching by the switching means.
An image pickup apparatus which is arranged as a further preferred embodiment of this invention to use, in common, a focusing device for both a moving-image shooting mode and a still-image shooting mode includes switching means for switching between the moving-image shooting mode and the still-image shooting mode, automatic focusing means for carrying out a focusing action by detecting a high frequency component of a video signal as a focus evaluating signal and by driving an image pickup system on the basis of the focus evaluating signal, and control means for controlling exposure control means in such a way as to have an iris almost fully opened when the focusing action is performed in the still-image shooting mode and to have the iris changed over to a required aperture value when a shot is taken in the still-image shooting mode after completion of the focusing action. With the apparatus arranged in this manner, when the moving image shooting mode is switched to the still-image shooting mode by the switching means, the focusing action is restarted by the restart means. A high frequency component of the video signal is detected as the focus evaluating signal. Then, the focusing action is again carried out by driving the image pickup system on the basis of the focus evaluating signal.
Further, when the shooting mode of the video camera is switched by the switching means which permits selection between the moving-image shooting mode and the still-image shooting mode, threshold value switching means changes over a focusing restart threshold value correspondingly with the shooting mode selected by the switching means. When the high frequency component detected as the focus evaluating signal becomes less than the changed-over focusing restart threshold value, the focusing action is restarted by driving the image pickup system.
Further, when the still-image shooting mode is selected by the switching means which permits selection between the moving-image shooting mode and the still-image shooting mode, the control means causes the iris to be set approximately at a full open position when the focusing action is performed in the still-image shooting mode and, when a shot is taken in the still-image shooting mode after completion of the focusing action, causes the iris to be changed over to a required aperture value.
The above and other objects and features of this invention will become apparent from the following detailed description of embodiments thereof taken in connection with the accompanying drawings.